Any Geo Metro with a stick shift in good running condition should return you 50mpg or more. Same with the Honda Civic VX. Both can be had for very little money (certainly under your $7k budget... Otherwise, a 2001-2003 Prius can be had for that price range, and also return 50mpg. I haven't looked into the TDI or Civic Hybrids enough to comment, but the TDI will return good MPG if you are on the highway.
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